Method and apparatus for improved long preamble formats in a multiple antenna communication system

ABSTRACT

Methods and apparatus are provided for improved long preamble formats in a multiple antenna communication system having N antennas. According to one aspect of the invention, a preamble having a legacy portion and a high throughput portion is transmitted (or received) on each of the N transmit antennas, wherein the legacy portion comprises a legacy long training field and the high throughput portion comprises at least N high throughput long training fields, wherein the N high throughput long training fields are transmitted in N time slots using an N×N orthogonal matrix. The orthogonal matrix can be, for example, one or more of a Walsh matrix and a Fourier matrix. The N time slots can optionally comprise a single symbol.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 60/711,025, filed Aug. 23, 2005, and is related toU.S. patent application Ser. No. 11/507,389, entitled “Method andApparatus for Reducing Power Fluctuations During Preamble Training in aMultiple Antenna Communication System Using Cyclic Delays,” and U.S.patent application Ser. No. 11/507,390, entitled “Method And ApparatusFor Improved Short Preamble Formats In A Multiple Antenna CommunicationSystem,” each incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to multiple antenna wirelesscommunication systems, and more particularly, to preamble trainingtechniques for a multiple antenna communication system.

BACKGROUND OF THE INVENTION

Multiple transmit and receive antennas have been proposed to provideboth increased robustness and capacity in next generation Wireless LocalArea Network (WLAN) systems. The increased robustness can be achievedthrough techniques that exploit the spatial diversity and additionalgain introduced in a system with multiple antennas. The increasedcapacity can be achieved in multipath fading environments with bandwidthefficient Multiple Input Multiple Output (MIMO) techniques. A multipleantenna communication system increases the data rate in a given channelbandwidth by transmitting separate data streams on multiple transmitantennas. Each receiver receives a combination of these data streams onmultiple receive antennas.

In order to properly receive the different data streams, receivers in amultiple antenna communication system must acquire the channel matrixthrough training. This is generally achieved by using a specifictraining symbol, or preamble, to perform synchronization and channelestimation. It is desirable for multiple antenna communication systemsto co-exist with legacy single antenna communications systems (typicallyreferred to as Single Input Single Output (SISO) systems). Thus, alegacy (single antenna) communications system must be able to interpretthe preambles that are transmitted by multiple antenna communicationsystems. Most legacy Wireless Local Area Network systems based upon OFDMmodulation comply with either the IEEE 802.11a or IEEE 802.11g standards(hereinafter “IEEE 802.11a/g”). Generally, the preamble signal seen bythe legacy device should allow for accurate synchronization and channelestimation for the part of the packet that the legacy device needs tounderstand. Previous MIMO preamble formats have reused the legacytraining preamble to reduce the overhead and improve efficiency.Generally, the proposed MIMO preamble formats, for example, inaccordance with an IEEE 802.11n standard, include the legacy trainingpreamble and additional long training symbols, such that the extendedMIMO preamble format includes at least one long training symbol for eachtransmit antenna or spatial stream.

A number of frame formats have been proposed for evolving multipleantenna communication systems, such as MIMO-OFDM systems. In oneproposed MIMO frame format, each transmit antenna sequentially transmitsone or more long training symbols, such that only one transmit antennais active at a time. As the transmit antennas are switched on and off,however, the temperature of the corresponding power amplifier willincrease and decrease, respectively. Generally, such heating and coolingof the power amplifier will lead to “breathing” effects that cause thetransmitted signal to have a phase or magnitude offset, relative to thedesired signal.

It is therefore desirable to have a continuous transmission from alltransmit antennas to avoid temperature related signal “breathing.” Thus,in further proposed MIMO frame formats, orthogonality is maintainedusing cyclic delay diversity (CDD), Walsh coding or tone interleavingacross the different transmit antennas. The CDD short training symbol,however, cannot measure the received signal power with sufficientaccuracy. Thus, additional backoff is required in the RF chain andadditional dynamic range is required in the digitization process.Likewise, the tone interleaved design is not fully backwards compatiblewith a number of existing 802.11a/g devices that use short training fortiming synchronization or use time domain channel estimation.

In a system that does not include legacy devices, or desires greaterefficiency at the expense of losing backwards compatibility, thepreamble does not need to include fields intended for legacy devices.Thus, a short preamble format has been suggested for reducing theoverhead associated with the preamble in such environments. A needtherefore exists for improved long and short MIMO preamble formats andtraining techniques that provide reduced preamble overhead.

SUMMARY OF THE INVENTION

Generally, methods and apparatus are provided for improved long preambleformats in a multiple antenna communication system having N antennas.According to one aspect of the invention, a preamble having a legacyportion and a high throughput portion is transmitted (or received) oneach of the N transmit antennas, wherein the legacy portion comprises alegacy long training field and the high throughput portion comprises atleast N high throughput long training fields, wherein the N highthroughput long training fields are transmitted in N time slots using anN×N orthogonal matrix. The orthogonal matrix can be, for example, one ormore of a Walsh matrix and a Fourier matrix. The N time slots canoptionally comprise a single symbol.

A portion of the preamble on a first of the N antennas can optionally bedelayed relative to a transmission of the corresponding portion of thepreamble on a second of the N antennas, wherein the delay is anon-orthogonal amount to introduce variation across the preamblestransmitted on the N transmit antennas.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an exemplary MIMO transmitter;

FIG. 2 is a schematic block diagram of an exemplary MIMO receiver;

FIG. 3 illustrates a preamble design that is backwards compatible with802.11a/g legacy devices;

FIG. 4 illustrates a technique that transmits the legacy preambleportion on a single antenna, and transmits the high throughput preambleportion in a tone-interleaving fashion across different transmitantennas;

FIG. 5 illustrates a technique for CDD transmission of the legacyportion of the preamble of FIG. 3, with tone-interleaving acrossdifferent transmit antennas for the high throughput training portion;

FIG. 6 illustrates an alternative existing long preamble design, asproposed by the World-Wide Spectrum Efficiency (WWiSE) alliance;

FIG. 7 illustrates an existing short preamble design, as proposed by theWWiSE alliance;

FIG. 8 illustrates the high throughput long training field (HT-LTF) ofthe preamble design of FIG. 3 in further detail;

FIG. 9 illustrates the HT-LTF of the preamble design of FIG. 6 infurther detail;

FIGS. 10A through 10C illustrate the HT-LTF of the preamble design ofFIGS. 6 and 9 in further detail;

FIG. 11 illustrates a HT-LTF incorporating features of the presentinvention;

FIG. 12 illustrates a long preamble format including the HT-LTF of FIG.11;

FIG. 13 illustrates a short preamble format incorporating features ofthe present invention for an exemplary four antenna system;

FIG. 14 illustrates an exemplary technique for transmission of the shortpreamble format of FIG. 13;

FIG. 15 illustrates exemplary content for illustrative high throughputsignal fields (HT-SIG1 and HT-SIG2) for the long preamble format of FIG.12; and

FIG. 16 illustrates an exemplary technique for autodetection of the longand short preambles disclosed herein.

DETAILED DESCRIPTION

The present invention provides long and short preamble formats andtechniques for preamble training for MIMO systems. FIG. 1 is a schematicblock diagram of a MIMO transmitter 100. As shown in FIG. 1, theexemplary two antenna transmitter 100 encodes the information bitsreceived from the medium access control (MAC) layer and maps the encodedbits to different frequency tones (subcarriers) at stage 105. For eachtransmit branch, the signal is then transformed to a time domain waveform by an IFFT (inverse fast Fourier transform) 115. A guard interval(GI) of 800 nanoseconds (ns) is added in the exemplary implementationbefore every OFDM symbol by stage 120 and a preamble of 32 μs is addedby stage 125 to complete the packet. The digital signal is thenpre-processed at stage 128 and converted to an analog signal byconverter 130 before the RF stage 135 transmits the signal on acorresponding antenna 140.

FIG. 2 is a schematic block diagram of a MIMO receiver 200. As shown inFIG. 2, the exemplary two antenna receiver 200 processes the signalreceived on two receive antennas 255-1 and 255-2 at corresponding RFstages 260-1, 260-2. The analog signals are then converted to digitalsignals by corresponding converters 265. The receiver 200 processes thepreamble to detect the packet, and then extracts the frequency andtiming synchronization information at synchronization stage 270 for bothbranches. The guard interval is removed at stage 275. The signal is thentransformed back to the frequency domain by an FFT at stage 280. Thechannel estimates are obtained at stage 285 using the long trainingsymbol. The channel estimates are applied to the demapper/decoder 290,and the information bits are recovered.

FIG. 3 illustrates a long preamble design 300 that is backwardscompatible with 802.11a/g legacy devices. The preamble design 300provides a dedicated legacy portion 310 with a signal field for backwardcompatibility and a dedicated high throughput (HT) MIMO training portion320 for decoding the high throughput payload. The legacy short trainingfield (L-STF) is typically used for power estimation, coarse frequencyoffset estimation, and coarse time estimation. The legacy long trainingfield (L-LTF) is used for fine time estimation and channel estimation inlegacy systems. The legacy signal field (L-SIG) is used, for example, toidentify the data rate at which the legacy payload has been transmitted.

Generally, as discussed further below in conjunction with FIGS. 4 and 5,the legacy portion 310 can be transmitted on a single antenna, or withCDD across the transmit antenna array. For a detailed discussion of animplementation of the preamble design of FIG. 3 and techniques foremploying CDD or tone-interleaving across different transmit antennas,see, for example, U.S. patent application Ser. No. 11/043,025, filedJan. 24, 2005, entitled “Method And Apparatus For Preamble Training In AMultiple Antenna Communication System,” incorporated by referenceherein.

In the exemplary preamble design 300, the transmitter 100 firsttransmits the legacy 802.11a/g preamble 310, for example, using CDD. Thelegacy preamble 310 permits the performance of packet detection andcoarse frequency offset estimation. The results of these two functionsare also going to be used in the MIMO transmission. In addition to thesetwo functions, the legacy preamble 310 is used to perform legacy AGC,timing and frequency synchronization and channel estimation, in a knownmanner. The receiver 200 then decodes the subsequent legacy and HTsignal fields. The HT signal field is also transmitted using CDD (ortone interleaving).

As shown in FIG. 3, following the legacy and HT signal fields is a MIMOshort training field (HT-STF) and then the MIMO long training fields(i.e., HT-LTFs). Each field is a logical connection of OFDM symbols. TheMIMO short training field (i.e., HT-STF) helps refine the AGC settingfor receiving a MIMO payload. It is also at this instance that thetransmitter may commence beam steering, if desired. The length of theHT-STF can be much shorter than the legacy short training field.

In an exemplary two antenna system, the HT-LTF is 7.2 μs. In anexemplary four antenna system, the HT-LTF is 19.2 us. In an exemplarysystem, the HT-STF is 2.4 μs long. In another exemplary system, theHT-STF is 4.0 μs long.

The HT-STF and HT-LTF fields of FIG. 3 can be constructed in atone-interleaved fashion, as discussed further below in conjunction withFIG. 4.

FIG. 4 illustrates a technique whereby the legacy portion of thepreamble is transmitted on a single antenna, and the high throughputportion of the preamble is transmitted in a tone-interleaving fashionacross different transmit antennas. As shown in FIG. 4, the legacyportion 310 is initially transmitted by first transmitter 410-1 in theMIMO system. Thereafter, in one exemplary embodiment shown in FIG. 4,the high throughput training portion 320 is transmitted by each antenna410-1 through 410-4 at a reduced power, such as a reduction of 6 dB,relative to the legacy portion 310, using tone interleaving. Generally,for tone interleaving, different tones of the high throughput trainingportion 320 are transmitted on different transmit antennas, such that asubcarrier (tone) is active on only one transmit branch 410 at a time.

The tone interleaving embodiment shown in FIG. 4 is backwards compatiblewith legacy devices. If the transmitter is power amplifier limited, therange may be compromised. In addition, even if the transmitter is notpower amplifier limited, there will be a 6 dB power ramp, which willrequire introduction of a guard time prior to the HT-STF else it woulddegrade the performance of the HT-STF.

FIG. 5 illustrates a technique for CDD transmission of the legacyportion 310, with tone-interleaving across different transmit antennasfor the high throughput training portion 320. As shown in FIG. 5, thelegacy portion 310 is transmitted across each transmit branch 510 withCDD. Generally, a signal is transmitted with CDD by putting the last Δsamples of the OFDM symbol to the beginning. As shown in FIG. 5, eachdifferent antenna has a different cyclic delay. The high throughputtraining portion 320 is transmitted by each antenna 510-1 through 510-4at full power, using tone interleaving.

The CDD embodiment shown in FIG. 5 does not demonstrate a power jump,and no power amplifier switching is required. The CDD embodiment shownin FIG. 5 is not limited by the power amplifier design. The CDD valuesare intended to introduce randomization (as opposed to orthogonalitywith a cyclic delay equal to ½ the OFDM symbol length) and the valuesare generally on the order of 5-10% of the OFDM symbol duration which is800 ns long. The CDD values, however, have to be chosen such that legacycompatibility is not compromised. The purpose of using the CDD techniqueis to mitigate unintended power variations in the far field.

FIG. 6 illustrates an alternative existing long preamble design 600, asproposed by the World-Wide Spectrum Efficiency (WWiSE) alliance. WWiSEis an alliance of companies developing a proposal for the IEEE 802.11nWireless LAN standard. Generally, the preamble design 600 is said to use½ symbol long cyclic delays (CDD) across the HT-LTF, as shown in FIG. 6,to maintain orthogonality among the preambles on the various transmitbranches 610. The “reserved bit” in the L-SIG field can be used toindicate a HT transmission.

FIG. 7 illustrates an existing short preamble design 700, as proposed bythe WWiSE alliance. The short preamble design 700 can be employed ifthere are no legacy devices present, backwards compatibility is nototherwise required or a more efficient preamble is desired. As shown inFIG. 7, the short preamble design 700 comprises a legacy short trainingfield, immediately followed by the high throughput portions of thedesign 600. In other words, the legacy long training and legacy signalfields are not part of the design 700. Again, as shown in FIG. 7,relatively large delay values (equivalent to ½ symbol lengths) areemployed in the HT-LTF to distinguish the long training fields on eachantenna 710.

The preamble 700 comprises two high throughput long training fields,that are sufficient to distinguish four transmit branches 710, when usedwith CDD and orthogonal precode mapping. In other words, for theexemplary design 700 of FIG. 7, ½ symbol cyclic shifts are used toresolve two spatial streams 710, and Walsh encoding can be used toresolve two additional spatial streams 710.

If the long preamble formats 300, 600 of FIGS. 3 and 6 are compared, itis apparent that the field allocations of the long preamble formats 300,600 are substantially similar, other than the fourth and fifth fields.In particular, the positions of the high throughput signal fields (i.e.,HT-SIG) are alternated in the two formats 300, 600, and the format 300employs a HT-STF in the fifth position, while the format 600 employs anadditional HT-LTF in the fourth position.

FIG. 8 illustrates the HT-LTF of the preamble design 300 of FIG. 3 infurther detail. As shown in FIG. 8, the first time slot of the HT-LTFcontains a repeated long training symbol and each subsequent time slotcontains only one long training symbol. In addition, as indicated above,all spatial streams are distinguished via tone interleaving. Thus, nosmoothing is required. The length of the HT-LTF for four spatial streamsis {(4*0.8)+(5*3.2)} or 19.2 us. If the LTF is not repeated in the firsttime slot, then the overall length (duration) of the HT-LTF of FIG. 8becomes 16 us.

FIG. 9 illustrates the HT-LTF 900 of the preamble design 600 of FIG. 6in further detail. As shown in FIG. 9, each time slot of the HT-LTFcontains a repeated long training symbol. Thus, additional estimation offine frequency offset (FO) is possible. For a two transmit antennasystem, orthogonality is maintained via a ½ symbol shift (using the CDDdelay amounts shown in FIG. 9). For the next two spatial streams in afour transmit antenna system, orthogonality is achieved via a 2×2 Walshencoding matrix (using the polarities “++” and “+−” across two timeslots, as shown in FIG. 9). For example, the first spatial stream isdistinguished from the second spatial stream by the ½ cyclic delay, andthe first spatial stream is distinguished from the third spatial streamby Walsh encoding across the two time slots. Since only two time slotsare employed, two Walsh codes are available, and two spatial streams canbe distinguished.

In the four transmit antenna implementation, two time slots are requiredto estimate the four spatial streams. It has been observed thatsmoothing is problematic if beam steering is performed at thetransmitter. The length of the HT-LTF of FIG. 9 is 16 us.

FIGS. 10A through 10C illustrate the HT-LTF of the preamble design 600of FIGS. 6 and 9 in further detail. FIG. 10A illustrates the 4×4 channeltransfer matrix 1010 for a four transmit and receive antennaimplementation. In addition, FIG. 10A illustrates the corresponding longtraining sequence 1020, based on the inverse Walsh matrix and CDD delayvalues shown in FIG. 9.

As shown in FIG. 10B, the first step is to extract the block vectors viaWalsh processing. The received signal is characterized in matrix form1030 and the inverse Walsh matrix 1040 for the ½ cyclic delay is appliedat the receiver. Finally, as shown in FIG. 10C, the channel, h(t), isextracted via cross-correlation with the ½ shifted and non-shiftedHT-LTFs.

Long Preamble Format

FIG. 11 illustrates a HT-LTF 1100 incorporating features of the presentinvention. As shown in FIG. 11, the HT-LTF 1100 employs one time slotper spatial stream (transmit branch). For example, in a four antennasystem, four time slots are employed and a 4×4 Walsh matrix is availableto fully distinguish the four spatial streams. Thus, cross correlationwith HT-LTFs is not required. In addition, the HT-LTF 1100 does notrepeat the long training symbol in each time slot. Nonetheless, thefrequency offset (FO) estimation can still be performed off of thelegacy long training field. The CDD values shown in FIG. 11 are chosento introduce randomization among the preamble and thereby reduce thepower fluctuation. The exemplary delay values correspond to 0, ½, ¼ and¾ cyclic delay values. Again, these delay values are not large enough toprovide orthogonality but are intended to randomize the deterministicpreamble.

Since the OFDM symbol is not repeated in each time slot, the length ofthe HT-LTF 1100 for four spatial streams is (4*0.8)+(4*3.2) or 16 us. Ifthe OFDM symbol is repeated in each time slot, the system is morerobust, since two symbols with twice the energy are available to improvethe estimation (at the expense of longer preambles).

The four spatial streams in FIG. 11 are separated using Walsh Coding(and CDD), as opposed to tone interleaving. In an implementationemploying three spatial streams, a Fourier matrix can be employed sincea corresponding Walsh matrix is not known to exist.

FIG. 12 illustrates a long preamble format 1200 including the HT-LTF1100 of FIG. 11. As shown in FIG. 12, the legacy portion 310 employs CDDencoding for randomization purposes.

In addition, the long preamble format 1200 includes a HT-STF that istransmitted using tone interleaving 1220 across the antenna array.Finally, the HT-LTFs are transmitted using Orthogonal Coding 1230. Forexample, full Walsh coding can be employed for two and four spatialstreams, and Fourier coding can be employed for three spatial streams.It is noted that the long preamble format 1200 provides a HT-LTF foreach transmit antenna, and ignores the legacy LTF.

Short Preamble Format

As discussed above in conjunction with FIG. 7, the WWiSE allianceproposed a short preamble design 700. As shown in FIG. 7, a first HT-LTFis used for two spatial streams and a second HT-LTF is appended forthree or four streams. Thus, the two high throughput long trainingfields are sufficient to distinguish four transmit branches 710, whenused with ½ symbol length CDD and orthogonal precode mapping. In oneexemplary design 700, ½ symbol cyclic shifts are used to resolve twospatial streams 710, and Walsh encoding can be used to resolve twoadditional spatial streams 710. Beam steering cannot be employed, sincethe HT-LTFs require smoothing, i.e., cross-correlation with HT-LTFs.

FIG. 13 illustrates a short preamble format 1300 incorporating featuresof the present invention for an exemplary four antenna system. Amongother features, the short preamble format 1300 incorporates a legacypreamble having a legacy STF, LTF and signal field. The short preambleformat 1300 is thus backwards compatible and can operate in a mixed modeenvironment (comprised of legacy and MIMO devices).

As shown in FIG. 13, and discussed further below in conjunction withFIG. 14, a first portion 1310 of the short preamble format 1300 istransmitted using the first, “all ones” column of the Walsh matrix andCDD, and a second portion 1320 of the short preamble format 1300 istransmitted using the remaining columns (columns 2-4 for a four antennasystem) of the Walsh matrix and CDD. The first portion 1310 includes thelegacy portion and is modulated by a “+” Walsh code. Thus, a legacydevice can recognize the legacy preamble to maintain backwardscompatibility. The Walsh encoding provides full orthogonality for theLTF fields to permit separation of the spatial streams. In addition, thefull Walsh encoding permits beam steering, particularly under a MACcover. The CDD delays are meant to introduce randomization into thepreamble to reduce power fluctuation.

The high throughput signal field HT-SIG can be transmitted with two orfour extra sub-carriers being active (for example, using 54 or 56 of the64 available subcarriers). The duration of the exemplary short preambleformat 1300 exceeds the duration of the short preamble format 700 by 8us. This extra 8us, however, considerably increases the utility of theshort preamble, for example, by including the 4 us L-SIG field thatallows the format 1300 to be backwards compatible. Generally, the CDDvalues should be chosen to meet legacy interoperability requirements. Inaddition, the presence of the legacy long training field allows anestimation of the high throughput channel.

In addition, while the long preamble format 1200 of FIG. 12 employs aHT-LTF for each transmit antenna, and ignores the legacy LTF, the shortpreamble format 1300 employs the L-LTF as well as N−1 high throughputlong training fields for an N antenna system. Generally, the longtraining fields are used for MIMO channel estimation.

FIG. 14 illustrates an exemplary technique 1400 for transmission of theshort preamble format 1300 of FIG. 13. The Walsh codes 1410 and CDDdelays employed to distinguish each spatial stream are shown in FIG. 14.The smaller CDD delays (relative to the design 700), make the format1300 compatible with legacy devices and are intended to introducerandomization among the preamble. The full Walsh encoding in accordancewith the matrix 1410 ensures that orthogonality is maintained.

FIG. 15 illustrates exemplary content for illustrative high throughputsignal fields (HT-SIG1 and HT-SIG2) for the long preamble format 1200 ofFIG. 12. For each bit in the signal fields, the corresponding assignmentis shown in the tables.

When a HT device is receiving a transmission with a mixed mode preamble,it has no apriori knowledge if the transmission is a legacy or highthroughput transmission. Therefore, a mechanism is needed for the HTdevice to automatically detect the presence of absence of a HT portionof the preamble. FIG. 16 illustrates an exemplary technique forautodetection of the long and short preambles disclosed herein. As shownin FIG. 16, the decision is performed at a boundary 1610, usingconditions 1620 based on Q-BPSK and inverted pilots. All three preambleformats discussed herein (legacy, HT short, and HT long) are identicaluntil boundary 1610. There are several techniques to signal thecontinuation of the preamble. For example, the HT-SIG constellation canbe rotated by 90 degrees. This technique is called Q-BPSK. In addition,the polarity of the pilots can be inverted in going from the L-SIG fieldto the HT-SIG field. This technique is called inverted pilots. If onlyQ-BPSK is used, this signals the presence of a HT short preamble. Ifboth Q-BPSK and inverted pilots are used, this signals the presence of aHT long preamble.

System and Article of Manufacture Details

As is known in the art, the methods and apparatus discussed herein maybe distributed as an article of manufacture that itself comprises acomputer readable medium having computer readable code means embodiedthereon. The computer readable program code means is operable, inconjunction with a computer system, to carry out all or some of thesteps to perform the methods or create the apparatuses discussed herein.The computer readable medium may be a recordable medium (e.g., floppydisks, hard drives, compact disks, or memory cards) or may be atransmission medium (e.g., a network comprising fiber-optics, theworld-wide web, cables, or a wireless channel using time-divisionmultiple access, code-division multiple access, or other radio-frequencychannel). Any medium known or developed that can store informationsuitable for use with a computer system may be used. Thecomputer-readable code means is any mechanism for allowing a computer toread instructions and data, such as magnetic variations on a magneticmedia or height variations on the surface of a compact disk.

The computer systems and servers described herein each contain a memorythat will configure associated processors to implement the methods,steps, and functions disclosed herein. The memories could be distributedor local and the processors could be distributed or singular. Thememories could be implemented as an electrical, magnetic or opticalmemory, or any combination of these or other types of storage devices.Moreover, the term “memory” should be construed broadly enough toencompass any information able to be read from or written to an addressin the addressable space accessed by an associated processor. With thisdefinition, information on a network is still within a memory becausethe associated processor can retrieve the information from the network.

It is to be understood that the embodiments and variations shown anddescribed herein are merely illustrative of the principles of thisinvention and that various modifications may be implemented by thoseskilled in the art without departing from the scope and spirit of theinvention.

1. A method for transmitting data in a multiple antenna communicationsystem having N transmit antennas, said method comprising the step of:transmitting on each of said N transmit antennas a preamble having alegacy portion and a high throughput portion, wherein said legacyportion comprises a legacy long training field and said high throughputportion comprises at least N high throughput long training fields,wherein said N high throughput long training fields are transmitted in Ntime slots using an N×N orthogonal matrix; and delaying a transmissionof at least a portion of said preamble on a first of said N antennasrelative to a transmission of said at least said portion of saidpreamble on a second of said N antennas, wherein said delay is anon-orthogonal amount to introduce variation across said preamblestransmitted on said N transmit antennas.
 2. The method of claim 1,wherein said orthogonal matrix is one or more of a Walsh matrix and aFourier matrix.
 3. The method of claim 1, wherein each of said N timeslots comprise a single symbol.
 4. The method of claim 1, wherein saidlegacy preamble is an 802.11 a/g preamble.
 5. A multiple antennacommunication system comprising: N transmit antennas for transmitting apreamble having a legacy portion and a high throughput portion, whereinsaid legacy portion comprises a legacy long training field and said highthroughput portion comprises at least N high throughput long trainingfields, wherein said N high throughput long training fields aretransmitted in N time slots using an N×N orthogonal matrix, wherein atransmission of at least a portion of said preamble on a first of said Nantennas is delayed relative to a transmission of said at least saidportion of said preamble on a second of said N antennas, wherein saiddelay is a non-orthogonal amount to introduce variation across saidpreambles transmitted on said N transmit antennas.
 6. The multipleantenna communication system of claim 5, wherein said orthogonal matrixis one or more of a Walsh matrix and a Fourier matrix.
 7. The multipleantenna communication system of claim 5, wherein each of said N timeslots comprise a single symbol.
 8. The multiple antenna communicationsystem of claim 5, wherein said legacy preamble is an 802.11 a/gpreamble.
 9. A method for receiving data in a multiple antennacommunication system having N receive antennas, said method comprisingthe step of: receiving on each of said N receive antenna a preamblehaving a legacy portion and a high throughput portion, wherein saidlegacy portion comprises a legacy long training field and said highthroughput portion comprises at least N high throughput long trainingfields, wherein said N high throughput long training fields are receivedin N time slots using an N×N orthogonal matrix, wherein at least aportion of said preamble on a first of said N antennas is delayedrelative to said at least said portion of said preamble on a second ofsaid N antennas wherein said delay is a non-orthogonal amount tointroduce variation across said preambles transmitted on said N transmitantennas.
 10. The method of claim 9, wherein said orthogonal matrix isone or more of a Walsh matrix and a Fourier matrix.
 11. The method ofclaim 9, wherein each of said N time slots comprise a single symbol. 12.The method of claim 9, wherein said legacy preamble is an 802.11 a/gpreamble.
 13. A multiple antenna communication system, comprising: Nreceive antennas for receiving a preamble having a legacy portion and ahigh throughput portion, wherein said legacy portion comprises a legacylong training field and said high throughput portion comprises at leastN high throughput long training fields, wherein said N high throughputlong training fields are received in N time slots using an N×Northogonal matrix, wherein at least a portion of said preamble on afirst of said N antennas is delayed relative to at least said portion ofsaid preamble on a second of said N antennas, wherein said delay is anon- orthogonal amount to introduce variation across said preamblestransmitted on said N transmit antennas.
 14. The multiple antennacommunication system of claim 13, wherein said orthogonal matrix is oneor more of a Walsh matrix and a Fourier matrix.
 15. The multiple antennacommunication system of claim 13, wherein each of said N time slotscomprise a single symbol.
 16. The multiple antenna communication systemof claim 13, wherein said legacy preamble is an 802.11 a/g preamble.